1. Field of the Invention
The present invention relates generally to infrared imaging systems and more specifically to a realtime, low distortion infrared imaging system capable of producing highly accurate thermal and spatial resolution images in more than one spectral band.
2. Background Art
All matter continuously emits and absorbs electromagnetic radiation. The infrared region of the electromagnetic spectrum can be conveniently divided into three regions called the "near infrared region" from 0.72 to 1.5 micrometers, the "intermediate infrared region" from 1.5 to 20 micrometers, and the "far infrared region" from 20 to more than 1,000 micrometers. These limits are somewhat arbitrary and result from the use of rather different detection devices in each of the regions.
In the near infrared region, images may be produced by infrared photographic emulsions. In the intermediate infrared region photoconductive and photovoltaic cells may be used to form images either through the use of scanning optics linear or mosaic arrays. The technology of infrared detectors over the last few decades has largely been a study in extending the wavelength response of infrared detectors.
The ideal absorber of infrared radiation, which is also an ideal radiator, is called a "blackbody". Any object that deviates from a perfect absorber or emitter is called a "non-blackbody radiator". All real world objects are such radiators although some, for example, lamp black, approximates an ideal blackbody.
The absorptance or emissivity of a substance in the visible region of the electromagnetic spectrum is no guide to its emissivity in the infrared. For example, a particular white paint can have a low emissivity in the visible, but be nearly equivalent to an ideal blackbody at wavelengths beyond 3 micrometers. This explains why an object covered with such a white paint would stay relatively cool in sunlight. It not only reflects much of the sunlight in the visible region of the spectrum, but it also reradiates the energy it has absorbed in the infrared region nearly as well as a blackbody. This is a common technique used to control the thermal balance of artificial satellites in space. A thorough discussion of the physics of infrared radiation may be found in "The Infrared Handbook", edited by Bill Wolfe and George Zissis, available from the Infrared Information and Analysis Center, P.O. Box 618, Ann Arbor, Mich. 48107.
Since World War II the ability to produce infrared images has gained great commercial and military application. An infrared imaging system is generally intended to provide a visual display that reproduces a scene as viewed in the infrared, whether it be on a film or by means of mechanical or electronic scanning.
Prior art infrared imaging systems may be divided into three types:
(1) Electrically scanned systems that use infrared sensitive imaging tubes such as is shown in U.S. Pat. No. 4,191,967 (a pyroelectric tube) and U.S. Pat. No. 4,142,206 (showing a pyroelectric solid state imaging device). These imaging tubes have low resolution and are noisy. Prior art systems using such sensing tubes have no inherent temperature accuracy and provide only relative radiance information.
(2) Mechanically scanned systems such as those illustrated in U.S. Pat. No. 4,193,688 (which shows a porro prism scanning separately energizible detectors). These mechanical systems, because they use solid state detectors, can see further into the far infrared (the 3 to 12 micrometer band being most usable), but they are extremely slow. Additionally, mechanical systems use a large complex of highly precise, moving optical surfaces to collimate and scan the field of view. These mechanical scanning systems typically introduce a great deal of either or spatial angular distortion into the image. Like electronic scanners, most prior art mechanical scanning systems only images relative radiance.
(3) Infrared sensitive photographic emulsions may be of high resolution, but are limited in spectral and radiometric sensitivity and must be chemically processed, which makes them unless for realtime systems.
One class of infrared images, called forward-looking infrared (FLIR) sensors, mechanically scan object space and reproduce the image using an array of infrared detectors. The geometric distortions introduced by the mechanically fixed and scanning optics, combined with the electronic noise and signal distortions caused by multi-channel electronics, produce poor quality infrared images. This approach was adopted because of the great increase in speed required to look at the infrared world in real-time, usually for military purposes, to make decisions such as those required for the tracking and firing of weapons systems. Thus, a certain amoung of geometric distortion in the image may be acceptable. Likewise, the intrernal sensor background noise and streakiness associated with FLIR system due to the fact that the multi-channel electronics on the different detectors are not identical, are acceptable in some of these military applications. As a consequence of current art, the FLIR's parallel detector channels also have different reference signal points and system gains, i.e. they are not normalized. The current art FLIR, thus, inherently introduces both temperature (i.e., electronic noise and lack of absolute standards) and spatial inaccuracies into the image is displays.
Some attempts have been made to overcome these limitations. The attempts made by the present state of the art, however, do not go in the direction of the present invention, but rather toward such systems as are taught by U.S. Pat. No. 4,121,248, which teaches a streak reduction electronic processing system for a FLIR display. Likewise, U.S. Pat. No. 4,214,271 teaches a technique for DC restoration and an AC-coupled display system. Similarly, Report No. TREE8050 from Purdue University in Lafayette, Indiana entitled "A FLIR Target Detection Algorithm" Final Report, November 1980, by Tom Huang, teaches the use of digitizing and computer processing an image in an attempt to compensate by past-analysis the inherent defects of the FLIR infrared imaging system. It should be emphasized that many millions of dollars have been and are continuing to be spent to try to overcome these basic limitations of the FLIR system. None of these prior art solutions to the problem, however, address the electronic and geometric distortions, from a casual viewpoint, at the sensor and scanner level; they attempt to process the noisy and distorted signal back into some semblance of reality.
Thus all prior art devices are either slow, for example, infrared photographic emulsions or line-scanned devices such as the THERMISCOPE.TM. as is taught by U.S. Pat. No. 3,631,248. Or they introduce temperature distortions within the image due to the difficulties of making multiple (up to several hundred) separate detector and electronic signal processing trains identical. Or they introduce spatial distortion due to the problems with present infrared reflective and/or refractive optics. Additionally, all present FLIR systems display only relaive radiance. Due to a lack of internal thermal reference standards, all FLIR prior art systems also have an inherent lack of thermal/radiance relative stability over time. All of these defects hold true even for slow systems that are mechanically or electronically scanned. When the speed of the system is increased to a point that it can be useful for realtime applications, these problems grow so severe that a vast and arcane art has developed to process deficient and distorted signals to make them the resulting image appear to be of higher quality. The problem unresolved by the prior art, is how to make a fast realtime infrared imaging system capable of producing highly accurate thermal and spatial resolution images.
An ideal infrared imaging system, as compared to present systems, would:
(1) Be spatially distortionless. (Present systems introduce mechanical scanning distortion.)
(2) Produce highly accurate thermal images that are referenced to the absolute radiance of a sensed standard (present FLIR systems display only relative radiance, they are not referenced).
(3) Normalize the output of each channel in a multichannel system against an internal reference at the midpoint of the imaged radiance. (Most present systems use no reference or clamp all channels to an arbitrary or average reference that does not allow for differences in detector response curves).
(4) Image two or more spectral bands in realtime.
(5) Interface with realtime video display or computer data acquisition systems with minimum storage or interface electronics (present systems require extremely complex delay line networks or large computer memories to store digitalized images).
As was mentioned above, neither the prior art, nor the present state of the art suggest being able to do all these things at one time. In fact, the present trend in the art is to accept basic problems as inevitable and to either use brute force engineering to minimize them or to process the signals produced by the prior art system to make the image look better. These "fixes" are necessary because the prior art does not treat the basic sensor and scanner problems stated above, but rather attempts to somehow minimize the symptoms caused by the problems.
For various reasons it is sometimes highly desirable to look at an infrared image in more than one portion of the infrared spectrum. For example, you might want to look at the infrared image in the 3 micrometers spectral band and then look at the same image in the 12 micrometers spectral band. In the past the only way to accomplish such an end would be either to use two separate systems and combine their output or to timeshare some elements of the system, usually the detector array. See, "IR System with Dual Field of View, Timeshare Processing of Two Images Uses Single Detector Array to Reduce Cost and Weight", National Technical Information Service Tech. Note No. G322SL3 (1980).
The state of the art is looking at different spectral regions may be found in U.S. Pat. No. 4,027,159, which teaches the combined use of visible and near infrared imaging with a far infrared detector that does not produce an image.
It is therefore an object of the present invention to provide a realtime infrared imaging system that is capable of rendering an accurate and noise-free thermal image without geometric distortion and with minimal noise.
Another object of the present invention is to provide an infrared imaging system capable of realtime observation of more than one spectral band.
Yet another object of the present invention is to provide a realtime infrared imaging system capable of imaging the actual energy flux or radiance emitted by a thermal source.
Yet another purpose of the present invention is to provide a realtime infrared imaging system whose output is electrically compatable with a video display or computer with minimum interface electronics.